- When we observe what we call fire
we have this combustion reaction going on
and then we see these flames
what we're really observing
are the three forms of thermal energy transfer.
We're observing conduction, conduction,
convection, convection,
convection,
and thermal radiation all at the same time.
So I'll do this, thermal, thermal radiation,
thermal radiation.
And I could say thermal conduction,
thermal convection, and thermal radiation,
and the word thermal is just relating to things
dealing with temperature.
So what is the conduction going on?
Well what we have going on in the fuel for our fire,
so down here the fuel for our fire
we have our classic combustion reaction
and I encourage you to watch the video on that
in our chemistry playlist, or chemistry section
if you're interested in it.
But you're taking carbon molecules and their bonds
and then in the presence of oxygen and some heat
you have a combustion reaction
producing carbon dioxide and producing water
and even more energy than you put into it,
so it's producing a lot of energy.
And that energy is going to excite
the molecules that are around it,
so you have these molecules,
you have these molecules
that end up getting a lot of kinetic energy.
And remember, temperature is proportional
to average kinetic energy.
And so the ones that just combusted,
or the molecules right near the reaction,
or maybe the ones that were part of the reaction,
they're gonna have this really high,
they're gonna have this really high average kinetic energy
'cause all of this energy was released
from that combustion reaction.
And then they're going to bump into other molecules
that might not have quite as high kinetic energy,
but then they're going to transfer that kinetic energy
through these collisions.
And we have a whole video on thermal conduction,
but that's what's happening.
Things with higher kinetic energy
are bumping into things with lower kinetic energy
and transferring some of that kinetic energy
and they're transferring some of that momentum.
So that is definitely happening
when we look at a fire like this,
that neighboring molecules are bumping
into other molecules and transferring energy.
Now convection, this is around the idea
that hot air is less dense.
So if we have a bunch of air particles here,
so let's say this is the cool air up here,
and it makes sense why cool air
is going to be more dense than hot air.
'Cause hot air, these particles have a much higher
kinetic energy and so they're gonna bump
into each other and they're gonna push each other
much further away and get more seperation
from each other because of that high average kinetic energy.
But because the hot area is less dense,
this area right over here is less dense, less dense,
than the white area, these might be all the same molecule,
I just made them in different colors to show
the less dense area and the more dense area,
it is going to rise, it is going to rise.
Or you can even think of it
as a more dense area.
The more dense area is gonna fall around it,
it's gonna fall around it, or try to go under it,
because it is more dense,
and then the less dense area is going to rise.
And by doing that,
you have the hotter molecules are moving upwards
and then the cold molecules can go down
to maybe the source of heat, the source of heat,
the source of energy right over here
to get heated up more.
So this right over here is, so hot air rises,
let me write this, the hot, hot air rises.
And when we think about it
it makes room for the cool air to come down
and then get heated up again.
And that's what's going on in the fire here.
You see the combustion reaction,
combustion reaction occurs right over there,
let me just hit a different color,
combustion reaction is occurring in the fuel,
that super hot air, it's going to rise.
And that's why it looks like these flames
are kind of, they're moving upwards,
they're kind of flickering upwards.
And you'll also see, and actually we're gonna
talk about this in thermal radiation,
is that they also change color as they move upwards.
But this whole idea of fire moving upwards
and the hot air,
if you put your hand up here it's gonna be much hotter
than if you were to put your hand, say, right here,
even though in theory this is closer to the flame
than up here and that's because the hot air,
the hot air is rising and it's making room for cold air,
or cooler air, I guess I could say,
to come down here and be part
and get heated up again,
and then it would rise up again.
And so this convection,
this idea of the hot air rising
or the cold air falling,
this is another form of heat transfer.
Now the last form that we're observing
when we're looking at fire is thermal radiation.
And this is all around the idea
that if you have charged particles being accelerated
they're going to release electromagnetic radiation.
And so you might say, Wait, wait, charged particles,
where are those being accelerated here?
Well, we have these molecules that are
constantly being accelerated as they bump
into each other,
the one that's gonna transfer kinetic energy
from, let's say this one bumps into that,
it's going to accelerate that in different direction
and even this one might go in another direction
and acceleration is a change in velocity
so it could be the magnitudinal velocity
or your directional velocity.
So as they're colliding you have all of these
accelerational charged particles,
so let me write this down,
you have acceleration, acceleration
of charged particles, charged particles,
particles,
and then that releases, that releases,
electromagnetic radiation,
that releases electromagnetic radiation.
I know what you're thinking, Wait wait,
hold on a second, Sal.
Okay, I can buy that we have molecules
that are being accelerated,
but where is this charged,
where are you getting this charged particles business?
Well we have to remember these molecules
are made up of atoms and atoms are made up of
charged particles, are made up of protons and electrons.
And so as you accelerate these
and the more that you accelerate these
the more radiation you are going to release.
And you might say, Okay,
you said I'm observing that in fire,
where am I observing radiation?
Well just the very fact that you can see the fire,
the light emitted from the fire,
that is electromagnetic radiation,
that is electromagnetic radiation.
It's just the electromagnetic radiation
in the wavelengths that your eye
considers to be visible light,
or that your eye considers to be light.
Even the particles up here,
so even the particles up here
that are still quite hot,
they are also emitting electromagnetic radiation
because they're all bumping into each other
and their electrons and their protons
are all getting accelerated in different ways,
they're also releasing electromagnetic radiation,
but it is at a slower wavelength
than your eye is capable of perceiving as light.
If you had an infrared camera
you would see the flames being much larger.
You would see them go all the way, all the way up here.
And if you were to look at the flame closely
you would see down here right where the
combustion reaction is happening,
the flame looks blue.
And that's because the blue light
is higher energy light,
and that's because the particles are being accelerated
more down here,
and then it goes from blue to kind of a white
to a yellow, to a red or to an orange, to a red,
and then it, to your regular eyes, it disappears.
But everything, everything that has some temperature
is releasing electromagnetic radiation.
And you're like, Okay, well that's all fine,
I can see it but how is that a form of energy transfer?
Well, if you're ever sat next to a flame
you will feel the heat.
In fact, even if the air between you and the flame is cold
you would still feel like you're getting warm.
So if this is a flame right over here,
so that is fire, and let's say you have cold air,
cold air, let's say you're at a campfire right over here,
maybe it's 30, this air right over here is 30 degrees,
if you are standing right over here
you would still feel heat,
you would still feel like you're getting warmed up.
And that's because that electromagnetic radiation
is being emitted from the air particles
that we perceive as fire
and then that can actually excite particles
on your skin
and it will transfer energy to your skin
and so you feel like you are actually getting warmed up.
I remember once, this is kind of a strange story,
but I was on the highway
and there was a car on fire
and I was literally, we drove to the far lane
because it was on fire,
we're three lanes away from it
and it kind of exploded.
I don't think anyone was in it,
hopefully no one was in it,
but I remember right when it exploded
it was an intense, immediate heat that we felt
through the window of the car,
and that was electromagnetic radiation.
That was thermal radiation being released
by these accelerated particles
in the air around that explosion,
which we perceived as a an explosion, or fire,
but then it was warming up particles on my skin.